Maitotoxin, the most potent marine toxin, is known to induce the uptake and the accumulation of Ca2+ into cells. In cultured cardiomyocytes, we previously demonstrated that the addition of maitotoxin results in the rapid influx of extracellular Ca2+, which evolves in few minutes and is followed by signs of cell damage and death. In the present study, the effects of maitotoxin on the cytosolic levels of free Ca2+ ([Ca2+]i) and on the intracellular stores of energy were studied in cardiomyocytes isolated from 2-3 day old rats. In freshly isolated, suspended cardiomyocytes, loaded with the fluorescent Ca2+ probe fura-2, maitotoxin induced a dose dependent increase in (Ca2+)i (EC50 0.3 ng/ml), which occurred with a lag interval of less than a minute and represented the earliest detectable effect of the toxin. Maitotoxin (1 ng/ml) induced a marked, almost complete depletion of intracellular ATP, which reached its peak after 30 minutes and preceded the signs of cell damage and death. All the effects of maitotoxin were prevented by incubating the cells in a low-Ca2+ medium, or in the presence of high concentrations of the calcium channel blocker verapamil. It thus appears that the maitotoxin-induced cardiototoxicity is secondary to an inordinate influx of Ca2+; the depletion of ATP appears to play a main role in the irreversibility of the cell damage that follows an increase in the cytoplasmic Ca2+.